The function of the skeletal muscular system is to provide stability and to enable movement of the human body. Accurate and precise measurements of the muscle volume are therefore crucial for further understanding of different diseases, syndromes, and disorders such as muscular dystrophis, sport injuries, inflammatory myopathies, spinal cord injury or sarcopenia (muscle loss due to aging). When diagnosing sarcopenia, muscle strength tests combined with muscle volume measurements are needed. Associated to aging and the progression of sarcopenia, the composition of the muscles also changes and an increased fat infiltration occurs. However, the impact of the higher fat content inside the muscles is not yet fully understood. For improved understanding of the prevalence, onset, and progress of sarcopenia, new methods, including an accurate technique for measuring muscle volume, are needed. Another example where detailed and accurate knowledge of muscle volume and muscle composition is important is for whiplash-associated disorders (WAD). A higher fat infiltration in the neck muscles has been found in people with WAD, compared to healthy controls. A higher fat concentration in the quadriceps muscle associated to the fibromyalgia syndrome has also recently been found.
There exist many approaches for measuring human muscle mass or volume. Non-imaging methods are often highly variable as they are usually calibrated on young healthy adults. The current standard imaging method for the determination of muscle mass and its distribution is dual energy x-ray absorptiometry (DXA), which is rapid and readily available. However, DXA uses ionizing radiation and only enables analysis of two-dimensional projections of the body. Therefore, no detailed muscle group separation, or quantification of fat content within the muscle tissue, can be obtained using DXA.
A more accurate analysis can be made using tomographic methods, i.e. CT and MRI. Water-fat separated MRI, based on Dixon imaging techniques, enables a high soft tissue contrast, providing detailed measurements of the muscle volumes and fat infiltration. The drawbacks of MR imaging are its availability and cost. With current techniques, scanning the whole body with sufficient resolution for body composition analysis may be achieved in less than ten minutes. However, the workload of manually segmenting the muscle tissue within the whole body is far too great to be feasible in anything but very small studies. Even when using optimized semi-automatic methods, a single segmentation of the whole body muscular system may take several working days to complete. The development of robust automatic segmentation of muscle tissue is therefore needed in order to make MRI an attractive alternative for studying muscle tissue volume in larger studies.
Anatomical knowledge can be incorporated into a segmentation method by atlases, i.e. real or synthetic images with corresponding manually defined anatomical labels. This anatomical knowledge, i.e. the segmented atlases, can then be transferred to a new subject (target) by non-rigid registration of the atlas onto the target's images. However, due to large anatomical variation between subjects and technical difficulties such as placement of arms and legs during scanning, a single registration may not converge correctly everywhere. Therefore, most atlas-based techniques address limited parts of the body, such as the brain, which shows a relatively limited variation in shape and location of its anatomical structures.
Non-rigid registration methods maximize the similarity between two different images. The result will vary depending on the similarity measure. Two common measures are image intensity and local phase information. One example of an intensity-based method is the Demons algorithm. Another example that enables a phase-based similarity measure is the morphon method. Phase-based methods are insensitive to gradual intensity variations, which are common in MR images due to B0 and RF inhomogeneity. An additional feature of the morphon is its ability to deform the prototypes on different scales with different degrees of regularization, an important feature for whole body registration.
There currently exists no method capable of providing a comprehensive and accurate description of the human skeletal muscular system that both quantifies the bulk of the muscle tissue volume and separates the muscle tissue into different muscle groups. A solution would be an important tool for studies of the interaction between phenomena such as the development of muscular atrophy, intra-muscular fat infiltration, and disease progression in a wide range of conditions including sarcopenia and muscular dystrophies.